On September 7, 2024, about 1030, mountain daylight time, a Taylorcraft BC12-D airplane, N95078, sustained substantial damage when it was involved in an accident near Trojan, South Dakota. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that while enroute the engine sustained a partial loss of power. The pilot attempted to restore engine power by applying carburetor heat, adjusting the mixture, and checking the fuel control valve. The pilot’s troubleshooting efforts were unsuccessful, and the airplane was unable to maintain altitude. The pilot descended into a wooded valley and conducted a forced landing into the treetops. The airplane came to rest inverted and sustained substantial damage to the fuselage and both wings. Postaccident examination of the wreckage revealed that the hold-down nuts on the right magneto were loose, and the magneto could rotate freely by hand. A lead deposit was noted in the No. 1 cylinder top spark plug; however, it was not found to inhibit the function of the spark plug. No other preimpact mechanical malfunctions were identified. A review of the airplane maintenance logbooks revealed that the last 100-hour inspection of the engine occurred on October 6, 2023. The logbooks stated that during this inspection, the magnetos were timed, and the spark plugs were cleaned and gapped. The last annual inspection was on September 1, 2024, six days before the accident. According to the mechanic who conducted the last annual inspection, the ignition timing was checked, but he did not recall if he had adjusted the magnetos. He also stated that the airplane had operated about 5 to 10 hours between the completion of the last annual inspection and the time of the accident.